Lactic acid is widely applied not only to uses such as food and pharmaceuticals, but also to industrial uses as a monomer material for biodegradable plastics, and increasingly demanded. 2-Hydroxypropionic acid, that is, lactic acid, is known to be produced by fermentation by microorganisms, wherein the microorganisms convert substrates containing carbohydrates such as glucose into lactic acid. Lactic acid is divided into optical isomers, the (L)-isomer and the (D)-isomer, based on the configuration of the substituent to the carbon at the α position of carbonyl group. By appropriately selecting the microorganism for microbial fermentation, (L)- or (D)-lactic acid can be selectively produced, or lactic acid as a mixture of the (L)-isomer and the (D)-isomer (racemic body) can be produced.
Production of lactic acid by microbial fermentation is generally carried out while a pH appropriate for the microbial fermentation is maintained by addition of an alkaline substance (e.g., calcium hydroxide) to the culture medium. Most of the lactic acid produced as an acidic substance by the microbial fermentation is made to be present in the fermented broth as a lactic acid salt (e.g., calcium lactate) by the addition of an alkaline substance. When lactic acid is used as monomers for a plastic, the lactic acid to be used is preferably free lactic acid obtained by adding an acidic substance (e.g., sulfuric acid) to the fermented broth after completion of the fermentation. However, the lactic acid fermented broth obtained by microbial fermentation contains, other than lactic acid as the product of interest, organic acids and salts thereof, proteins, amino acids, and nonionic compounds such as glycerol, as impurities. When lactic acid is used as monomers for a plastic, the lactic acid needs to be separated from these impurities.
For example, as a method of removing impurities from an aqueous lactic acid solution derived from a lactic acid fermented broth obtained by microbial fermentation, Japanese Translated PCT Patent Application Laid-Open No. 2001-506274 describes a method in which an aqueous lactic acid solution is subjected to ion-exchange treatment to remove ionic components, and the processed liquid is then subjected to distillation. As a method of removing nonionic impurities, JP 2006-75133 A describes a method in which lactic acid contained in an aqueous lactic acid solution is adsorbed to, and then eluted from, an ion-exchange resin to remove glycerol, which is an impurity contained in the aqueous lactic acid solution (however, Examples in JP '133 do not describe an example in which lactic acid is adsorbed to, and then eluted from, an ion-exchange resin).
To separate lactic acid from glycerol, which is a nonionic impurity contained in an aqueous lactic acid solution, there is a method in which, as described above, lactic acid contained in the aqueous lactic acid solution is adsorbed to, and then eluted from, an ion-exchange resin to remove the impurity, glycerol. However, since allowing a large amount of lactic acid to be adsorbed to an ion-exchange resin requires a large amount of the ion-exchange resin, it is thought that there are problems such as requirement of large equipment.
It could therefore be helpful to provide a method of separating lactic acid from an aqueous lactic acid solution containing glycerol as an impurity, which method enables simple and low-cost production of lactic acid in which glycerol is reduced.